The present invention relates to a device for measurement of rotational speed or position. More particularly, a tachometer and angular position indicator for use on an electric motor is disclosed. The concept works on all electrical motors that exhibit counter EMF pulses present on the power connections. The device detects energy imparted to the power supply terminals of a motor by the counter electromotive force produced during commutation. The detection of this phenomena is harnessed to and used in various applications, including sensing of motor rotational speeds and angular positions.
Typically, shaft angular position and rotational speed of an electric motor or other similar rotational transducer is accomplished by use of analog synchronous or digital encoder devices which may be integrated internally to a motor assembly, but are most commonly mounted external to the motor on the output shaft.
Analog synchronous devices take advantage of the phenomena of induced current in a conductor from a time varying magnetic field. In such an arrangement, a device consisting of a magnetic material and an inductive pick-up coil of conductor is attached to a rotor of a motor. As the magnet rotates, the magnetic field sensed by the conductor coil varies with time and thus induces a current flow in the conductor. The rotational speed of the magnet and hence the rotational speed of the motor shaft can be determined by measuring this current.
Digital encoder devices include those which consist of a disk of alternately transparent and opaque material. This disk is connected to the shaft of a motor. A stationary light source is placed on one side of the plane of the disk, a light detection device is similarly placed directly opposite. As the motor turns the disk, an electrical pulse is generated by the light detection device each time a transition from transparent to opaque is encountered. Counting these transitions allows a determination of disk (and hence motor) position and speed.
For example, Schoonover et al. (U.S. Pat. No. 3,934,200) Jan. 20, 1976 disclose a tachometer circuit comprising a nonlinear DC tachometer which produces an output voltage having an AC ripple. After removing the DC component, the AC voltage ripple is inverted and isolated. The AC voltage ripple is used to establish a nonlinear ripple-free output voltage and a means for linearizing the nonlinear ripple free output voltage. This reference filters the counter-EMF signal which Applicant utilizes to advantage.
Another device is Liden (U.S. Pat. No. 3,858,109) filed Dec. 31, 1974 which discloses a brushless tachometer. An alternating current variable permeance resolver is mechanically coupled to rotate in synchronism with a direct current brushless generator, the rotor of which is rotated at a speed desired to be measured. The generator produces two alternating current voltages. These two AC voltages are modulated by an AC voltage the frequency of which is very high relative to the maximum design generator-resolver rotational speed. The voltage at the output winding is therefore proportional to the speed components of the generator signal. The output signal is then demodulated at the same modulation frequency thereby to provide a DC output signal proportional to the rate of rotation of the generator rotor.
Another example is Katsumura (U.S. Pat. No. 4,788,492) filed Nov. 29, 1988 which is a reference disclosing an apparatus for measuring and detecting the speed and position of a rotating shaft of an electric motor. A commutator is provided having three or more poles. First and second brushes are provided in sliding contact with the commutator for supplying electrical power to the poles of the commutator. At least one of the poles is in electrical communication with the shaft. Rotation detection means measure the electrical potential difference between the shaft and ground and generate a rotation information signal in response to the changes in the electric potential differences as the shaft rotates.
Still another specially constructed device of general interest to the field of motor speed and angular position measuring is Howlett (U.S. Pat. No. 4,689,532) filed Aug. 25, 1987. Therein a ferrite sensor self-control synchronous motor is disclosed. A self-controlled synchronous motor is provided with at least one armature winding, a rotor rotatably mounted for rotation in proximity to the winding and a magnet located on the rotor for providing a magnetic flux. A sensor wire is located within the vicinity of variations in magnetic field intensity produced by rotor rotation and provides an output electrical signal indicative of the position of the rotor. Counting voltage pulses provides a rotational velocity output.
Many of these devices traditionally provide extremely accurate measurement of shaft angular position and shaft speed, but at the penalty of physically modifying the motor shaft by mechanical connection. Some motors are equipped with extended shafts to accommodate these connections, but may are not. Many applications do not allow physical placement of such devices on motor shafts because of mechanical constraints such as size or shaft loading. These drawbacks as well as the restrictive expense of such devices often prohibit their use in some applications.